Among the elements of the main group, the first ionization energy increases

From left to right across a period.

Explanation:

The least amount of energy needed to withdraw the valence electron is known as the ionization energy. The alkaline metals exhibit lowest ionization energy and are removed easily, as they possess one valence electron, these metals are present in far left in the main group. Right next to alkaline metals, that is, the alkaline earth metals exhibit higher ionization energy in comparison to alkaline metals, as they exhibit two valence electrons. The nonmetals are present far right in the main group and they exhibit the highest ionization energy, as they possess numerous valence electrons.

in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".